1. Field of the Invention
This invention relates generally to data communications systems and more particularly to a transceiver system for transferring multiple symbol states at low bit-error rates (BERs) in a narrowband channel.
2. Description of the Related Art
Many useful methods are known in the art for transmitting digital data through a channel. Radio-frequency (RF) channels are allocated by the Federal Communications Commission (FCC) and licensed for use by various entities, including the general public. Much of the Very-High/Ultra-High Frequency (VHF/UHF) band available to the general public is allocated in 25 kHz channels and new spectrum allocations are unavailable from the FCC because of demand from many other competing users. This allocation scheme prevents using these narrowband channels for many high-bandwidth communications applications. As used herein, “narrowband” denominates a bandwidth of less than 0.1%, which is understood to include the standard 25 kHz VHF (30–300 MHz) and UHF (300–3000 MHz) channel because it occupies a bandwidth less than 0.1% of the carrier frequency. Adapting such standard narrowband channels for video and other multimedia applications requires some new method for transferring data in a single 25 kHz channel at bit rates above the 19–20 kbits/sec limits known in the art for mobile applications.
The conventional RF channel capacity is limited by the distortion and noise arising from the hardware used to transmit and receive the RF signal and presents two basic challenges. Unless the channel transmitter hardware can be made to operate linearly over the range of signal power levels, no more than two signal states (symbols) can be used to reliably represent the data transferred in the channel. Unless the signal-to-noise ratio (SNR) at the receiver is high enough, then the uncertainty (blurring) of the symbol states at the receiver is too small to permit data recovery without error. Both of these challenges have been separately addressed by many practitioners in the art.
Discrimination among more than two symbol states at the transmitter and receiver is a useful method for increasing the amount of data transferred over a single channel. Unfortunately, the data rates in most systems known in the art, especially those well-known for mobile applications, are limited by the nonlinear operation of their transmitter components, such as amplifiers, modulators, mixers, and common integrated devices. But even introducing expensive precision components to improve transmitter operational linearity has limited effect on channel capacity when significant output power levels are required. The RF output device impedances change drastically over the extreme output power fluctuations encountered in high Peak-to-Average Power Ratio (PAPR) systems. This can, for example, produce uncontrollable phase shifts up to 30° between the two power extrema in a digital phase shift keying (PSK) modulated output waveform, thereby severely limiting discrimination among more than two symbol states at the transmitter. Linear Class A amplifiers avoid these large variations, but Class A amplifiers are grossly power-inefficient, which is a critical issue in most mobile applications.
Several other useful methods for linearizing the output characteristic of a RF transmitter are known in the art. For example, the basic feedback principle was introduced by Black in the 1920's. Black showed that distortion in the forward branch is reduced by 1/T, where T is the loop gain of the amplifier feedback combination. Distortion in the feedback branch appears directly in the output. The fundamental problem with basic feedback is that it uses an output that already exists to correct the input that caused the output distortion, which works well only with periodic signals and suffers from stability and bandwidth problems and from transient intermodulation distortion with signals that exceed the feedback loop bandwidth.
Other well-known linearizing methods also suffer from associated disadvantages that limit their application to the channel capacity problem. For example, predistortion methods require adding a predistortion to the amplifier input signal to compensate for amplifier nonlinearity. But the compensating nonlinearity widens the transmitted signal spectrum, requiring higher sampling rates, wider intermediate-frequency (IF) filters and so forth. Linear amplification with Nonlinear Components (LINC) methods are also well-known in the art but suffer generally from the disadvantageous requirements of high component precision and effective signal separation.
At radio frequencies, it may be difficult to achieve very high loop gain with simple feedback. One way to circumvent this is to form the error signal at the baseband frequency and use quadrature up- and down-conversion to provide orthogonal baseband components (I&Q) in the direct and feedback branches for linearizing the output. This is denominated “vector feedback,” which is known in both Cartesian and polar feedback embodiments. But any mixer or amplifier device delays reduce the bandwidth achievable with this technique. Also, the achievable output linearity is directly reduced by any noise or non-linearity in the feedback mixers. Finally, over wideband channels, Cartesian feedback suffers from stability problems. However, these bandwidth and stability restrictions are largely offset by the simplicity and robustness of Cartesian feedback amplifiers, including embodiments using low-precision components.
Even with otherwise good SNR, discrimination among more than two symbol states at the receiver is limited by the phenomenon of multipath distortion, which may create interfering time-delayed copies of the original signal, distorting the received waveform and smearing the symbol content at the receiver. Many practitioners have proposed solutions to the wideband multipath distortion problem. Adaptive receivers for discriminating against selective in-band fading in wideband channels are known in the art but are not widely used because of high cost and complexity. Generally, a method of reducing the effective data rate is employed so that delayed versions of the original signal do not interfere at the receiver, at least on average, having decayed or decorrelated over a relatively long intersymbol time interval. One strategy for accomplishing this is to use multiple sub-carriers with a reduced data rate in each. For example, Ditzel et al. [Ditzel et al., “Minimal Energy Assignment for Frequency Selective Fading Channels,” International Symposium on Mobile Multimedia Systems & Applications (MMSA2000), Delft, NL, December 2000] describe a method for minimizing the total energy necessary to communicate data with a desired average bit error rate at a fixed gross bit rate over a frequency-selective fading channel using multi-carrier data transmission. Ditzel et al. describe a numerical approach for splitting the data among multiple sub-carriers to reduce individual data rates with particular advantages for multi-carrier communications systems in channels with deep fades.
Orthogonal Frequency Division Multiplexing (OFDM) modulation is a well-known and effective technique for combating selective in-band fading through decorrelation of symbol echoes. With OFDM modulation, the fast Fourier transform (FFT) is used to break a broadband channel with intersymbol interference (ISI) into several narrower subchannels, each exhibiting flat fading characteristics. Several multiuser OFDM schemes are known in the art, including time-division, frequency-division, and code-division multiplexing. Code-division multiplexing is an effective technique for capitalizing on the frequency diversity of an OFDM modulation system and frequency-division multiplexing with optimal subcarrier allocation is known to vastly outperform other multiuser techniques when the transmitter has knowledge of the channel.
The application of OFDM modulation to wideband channels suffers from several unresolved problems, which are described in excellent detail by Martone [Max Martone, “On the Necessity of High-Performance RF Front-ends in Broadband Wireless Access Employing Multicarrier Modulations (OFDM),” GLOBECOM 2000—IEEE Global Telecommunications Conference, no. 1, November 2000, pp. 1407–1411]. But Martone does not propose specific solutions to these problems other than noting that wideband OFDM modem throughputs are actually much lower than typically advertised for transceivers with low-performance analog RF circuits. In fact, Martone suggests that OFDM vulnerability to RF amplifier distortion is actually so severe that the “popularity of OFDM scheme should be revisited in light of practical RF implementation issues.” Even so, Martone considers only RF channels typically much wider than 0.1% of the carrier frequency. Other problems with OFDM modulation in wideband channels include, for example, the severe symbol recovery problems presented when the channel has inband nulls or severe fades close to the FFT grid.
Various solutions are proposed for such problems. For example, Wang et al. [Wang et al., “Linearly Precoded or Coded OFDM against Wireless Channel Fades?” Third IEEE Signal Processing Workshop on Signal Processing Advances in Wireless Communications, Taoyuan, Taiwan, Mar. 20–23, 2001] propose adding a linear precoding to the symbols before multiplexing to overcome the loss of diversity otherwise experienced with critical fades. As another example, Wesolowski et al. [Wesolowski et al. “Efficient Algorithm for Adjustment of Adaptive Predistorter in OFDM Transmitter,” UTC'00, Fall, Boston, 2000] describes a useful predistortion technique for “linearizing” or compensating the RF amplifier distortion in OFDM transceiver system. Wesolowski et al. suggest that other techniques such as clipping to reduce the high PAPR and adding error correction coding do little to improve the system bit-error rate (BER). In U.S. Pat. No. 5,598,436, Brajal et al. disclose and claim a OFDM modulated communications system employing a predistorter circuit between modulator and RF amplifier to compensate for RF amplifier nonlinearity, thereby solving the multicarrier distortion problem by predistorting the OFDM subcarriers instead of the initial symbols. Brajal et al. specifically distinguish their invention over the earlier monocarrier systems and neither consider nor suggest methods for improving channel capacity in a narrowband channel.
In U.S. Pat. No. 5,914,933, Cimini et al. describe a method for wireless transmission of data in a wideband channel where the frequency response characteristic of each transmission sub-channel is first measured and fed back to the transmitter before distributing the digital data symbols over several clusters and switching each cluster to the transmission sub-channel that is optimal according to the measured frequency response. The method described by Cimini et al. spreads the symbols over many carriers within a wideband channel and does not suggest the use of any particular modulation or encoding methods. Cimini et al. neither consider nor suggest methods for better exploiting an existing narrowband channel.
OFDM modulation as not generally been applied to narrowband (less than 0.1%) channel applications, perhaps because of less concern for selective in-band fading. The usual benefit of using OFDM in a frequency-selective environment is that the effects of delay variation over the channel are minimized by dividing the transmitted bandwidth into many narrower sub-channels that are transmitted in parallel, thereby eliminating the need for an expensive channel equalizer. This also reduces the PAPR at the transmitter because fewer tones are transmitted per subchannel, producing less spectral spreading by any transmitter nonlinearities.
Practical OFDM modulation systems may employ, in each of the several subcarriers, quadrature phase shift keying (QPSK), eight-phase shift keying (8PSK), or some form of amplitude/phase shift keying (APSK) or quadrature amplitude modulation (QAM). Because APSK is merely a combination of amplitude shift keying (ASK) and PSK modulation, the term APSK modulation is intended to include PSK modulation and QAM, as used herein. It is known in the art that the error performance of APSK systems generally, for example, 8PSK and 32QAM systems, depends strongly on phase distortion from noise and system nonlinearities (see, for example, J. Pinto and I. Darwazeh, “Magnitude and Phase Distortion Effects on Error Vector Magnitude in 8-PSK Systems,” Proceedings of 3rd Conference on Telecommunications (Conftele-2001), Figuera da Foz-Portugal, pp. 351–355, April 2001). Disadvantageously, in many narrowband mobile system applications, high level APSK constellations are impractical because the amplitude of the signal (representing the symbol state) can fade to become indistinguishable from an adjacent symbol state. Wireless transmission systems using OFDM modulation are attractive because of their high spectral efficiency and resistence to noise and multipath distortion. But OFDM modulated transceiver systems disadvantageously require additional signal processing steps or pilot subcarrier symbols to achieve the BER of associated spread-spectrum techniques, thereby imposing a corresponding burden on useable channel capacity and limiting usefulness in narrowband channels. Moreover, the use of computationally-efficient FFT multiplexing methods generally requires an inter-symbol guard interval greater than the channel delay to avoid inter-symbol interference (ISI) and inter-channel interference (ICI), presenting yet another disadvantage for narrowband OFDM applications.
Generally, until now, there has been no method known in the art that can effectively transfer a data sequence through a single standard 25 kHz VHF/UHF channel at bit rates above about 19.2 kbits/sec in mobile applications. Commercial applications requiring more capacity use wideband channels at higher base frequencies in the cellular spectrum. There is accordingly a clearly-felt need in the art for a transceiver system that exploits more of the typical 0.1% narrowband (e.g., 25 kHz VHF/UHF) channel. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.